This invention relates to electroacoustic transducers such as microphones which may be integrated into a semiconductor substrate including other components.
Presently, demand is growing for a microphone which may be formed as part of an integrated circuit for such uses as telecommunications. Miniature microphones presently available usually take the form of a foil (which may be charged) supported over a metal plate on a printed circuit board so as to form a variable capacitor responsive to voice band frequencies. While the operation of such devices is adequate, they are quite distinct from the integrated circuitry with which they are used. A microphone which could be integrated with other components in an integrated circuit would be more compact, more economical to manufacture and ultimately have lower parasitics and better performance.
Recently, an integrated microphone structure and method of manufacture were proposed. (See U.S. patent application of I. J. Busch-Vishniac et al, Ser. No. 469,410, filed Feb. 24, 1983 and assigned to the present assignee, which application is incorporated by reference herein.) Briefly, the microphone included a membrane formed from a thinned portion of a thicker semiconductor substrate, which membrane had a thickness and area such that it vibrated in response to incident sound waves. A pair of electrodes formed a capacitor, and one of the electrodes was formed to vibrate with the membrane such that the capacitance varied in response to the sound waves and an electrical equivalent to the acoustic signal could be produced.
Such microphones offer considerable promise for the replacement of distinct miniature microphones previously described. However, with this or other types of integrated capacitive microphones, the dc bias available for integrated circuits limits the sensitivity of the microphone and places constraints on the size of the air gap (the separation of the electrodes). It has been suggested to charge an electrode of a microphone, thereby forming an "electret" (charged layer), which is combined with other components in an integrated circuit (see U.S. Pat. No. 4,149,095 issued to Poirier et al). However, there is apparently no previous teaching as to how a built-in bias could be provided in a completely integrated microphone.
It is therefore a primary object of the invention to provide an integrated electroacoustic transducer with a built-in bias, and a method of manufacturing such a structure which is compatible with integrated circuit fabrication techniques.